Humidity sensors are an important piece of equipment in the medical, automotive, HVAC, and food production industries, among others. Any industrial facility that requires a controlled atmosphere requires a properly functioning humidity sensor. Whether your requirements for maintaining or reducing humidity levels are for safety, function, environmental control, or comfort, it's important that you choose the most appropriate sensor for the job.

Choosing a Humidity Sensor

By knowing what questions to ask when choosing a proper humidity sensor for your situation, you can ensure successful operation and longer life for your equipment. The first step is to know your budget and assess your needs. Some of the factors to consider here include:

Size, accuracy, and interchangeability

Long-term stability needs

Repeatability requirements

Required resistances of the sensor to any chemicals or contaminants it will be exposed to

Resistive humidity sensors measure an electrical change that can be caused by a conductive substance. These systems use electrodes and AC excitation voltage to produce a data value for relative humidity.

Thermal conductivity humidity sensors measure absolute humidity. They do this by calculating the difference in the conductivity of dry air compared to air that contains water vapor. These sensors use thermistor elements to form a bridge circuit and measure the difference in resistance.

Selecting the Right Humidity Sensor for Your Application

Choosing the most appropriate humidity sensor for your application can help ensure the cost-effectiveness and success of your project.

Accuracy

The first aspect you should examine is the level of accuracy you need. The more accurate the sensor needs to be, the higher the cost will be.

Interchangeability

Cost can also be affected by the interchangeability of the sensor, which may cause vibration and lead to additional costs for calibration.

Condensation

If condensation is present, you may also need to factor in the cost of condensation resistant filters.

Packaging

Package type can impact cost savings if you choose a combined humidity and temperature sensor instead of two separate sensors.

Power

Power consumption will have overarching long-term effect on costs.

Different applications will have additional considerations that must be reviewed. Industrial applications have to consider the environment of the system and whether or not the sensor is resistant to chemicals or contaminants. For example, healthcare applications often require precise conditions and accurate, repeatable measurements, as well as the ability to maintain a clean environment. Similarly, humidity sensors are critical in HVAC and transportation applications to prevent excess moisture that fosters bacteria and mold growth.

Other Important Considerations

When considering the purchase of a humidity sensor, there are a few logistical concerns that also need to be reviewed. Ask the following questions to help you narrow down the proper solution and provider:

What are the lead times of the supplier and will you get your parts in time?

Does the supplier have a location close enough to provide in-person consultations and to deliver your product, or will the distance increase the lead time?

Do you have second source requirements in the event of a supply chain issue?

Can the supplier make a product unique to your specs or will they try to retrofit an existing product?

Are they willing to share data and guarantee their accuracy with a warranty and service policy?

Do they stand behind their products with technical support?

How does the price compare to the rest of the market when you look at the component, consultations, and long-term service provided?

Humidity Monitoring Solutions from Servoflo

As you can see, there are many factors to take into consideration when selecting a humidity sensor. Not only do you need to make sure you’ve selected the right one for your application, but you also need to make sure that you select the right company.

Servoflo has been providing sensor solutions to a broad range of industries and applications for nearly 20 years. To learn more about what it takes to select the best humidity sensor for your project, check out our eBook here.

Servoflo distributes a variety of capacitive and resistive humidity sensors for medical, HVAC, instrumentation, critical containment and other applications. Some situations need precise and highly accuracy relative humidity (rH) measurement to maintain narrow, specific environmental conditions while others have a broad temperature range with wider tolerance ranges for rH measurement. This blog post explains the differences between capacitive and resistive relative humidity sensors and why to choose one type over the other.

Capacitive Sensors

Humidity and dew sensors, also called hygrometers, are necessary when consistent environmental conditions are required to be maintained. A capacitive humidity sensor is so named because it uses capacitive measurement, which relies on electrical capacitance. These are a common type of humidity sensors.

The capacitive sensor is composed of two metal plates, or electrodes, separated by a thin layer of non-conductive polymer film. The film attracts moisture from the air and when the moisture makes contact with the metal plates it creates a voltage change. The output voltage measurement is captured and can be displayed via an analog dial, output into another system, or converted into a digital reading that indicates the amount of moisture in the air.

The capacitive sensor is contained in a hygrometer to measure relative humidity. The sensor is one part of the system that also contains a probe, cable, electronics, and an output signal. Together they function to accurately measure the entire humidity range from 0% to 100%.

Resistive Sensors

Resistive sensors work on a similar principle to capacitive sensors, where electrical change is measured to produce a value for relative humidity. However, the mechanism in this system is different. Although resistive sensors use a hygroscopic (moisture-absorbing) material similar to the capacitive system, the difference is that the measurement is of the resistance change in the material rather than the capacitance.

In this case, the output voltage has an inverse exponential relationship to relative humidity. As with capacitive sensors, the data regarding the output voltage can be captured, stored, or converted. These sensors use cost-effective, scalable dielectrophoresis, in which a free-standing structure is created to house pre-patterned electrodes that are separated by a single crystalline zinc oxide nanowire (ZnO NWs).

The benefit of a resistive sensor is the high surface-volume ratio, which allows it to measure humidity changes in the environment up to 90% relative humidity at room temperature. These systems do have limitations, and as such are not optimal for measuring values below 5% RH. Applications that favor a resistive sensor include automotive, smart food packaging, and relative humidity sensor networks.

Selecting a Humidity Sensor

Although the capacitive sensor and resistive sensor have the same goal in measuring humidity, they differ in their methods. Capacitive sensors are considered to be more accurate and stable while producing repeatable results—however, their cost can be prohibitive, making them more suitable for medical type applications where precision is paramount. The less expensive resistive sensors are more practical for situations where frequent measurements are taken that do not require extremely precise data.

For more detailed information about selecting a humidity sensor and the applications and industries these are frequently used in, download our eBook. You can check out our full line of digital humidity sensors on our website here.

For nearly 30 years, the team at Servoflo has been helping companies across a range of industries manage their unique measurement needs. Whether clients are working with standard pressure sensors, vacuum sensors, sensor signal conditioning solutions, oxygen sensors, or micropumps, we’ll work to find the ideal components for your specific measurement application.

Our vast inventory is always growing to meet shifting industry needs and incorporate the latest technologies, and we’re now proud to introduce a new line of digital humidity sensing products from MEMS Vision: smart sensors with unmatched accuracy and breakthrough manufacturing technology.

Microelectromechanical Systems: Tiny Tech With Major Impact

Microelectromechanical systems, or MEMS, are generally comprised of components less than 100 micrometers in size, and a standard MEMS device typically measures between 20 micrometers and 1 millimeter.

Scientists and industry leaders have been aware of the high potential of micro machines since the 1950s; manufacturing technology simply needed to catch up. For example, today’s MEMS are built with modified semiconductor device fabrication technologies, such as wet etching, molding, plating, dry etching, and electro-discharge machining.

The team at MEMS Vision specializes in innovative small devices and components, crafting some of the world’s leading nanotechnology. Their unique line of relative humidity and temperature (RH & T) products are made with breakthrough MoSiC™ micro/nanotechnology, and are designed to serve a wide range of original equipment manufacturer (OEM) applications. All sensors are backed by expert engineering techniques for optimal performance and minimal time to market.

The MVH3200D line of miniaturized digital humidity sensors, in particular, is engineered for unparalleled accuracy. These smart sensors can be independently programmed for customized humidity and temperature resolutions, providing maximum flexibility alongside full calibration. MVH3200D sensors can also provide standard digital I2C outputs to enable plug-and-play integration.

The micro-Watt levels of power demanded by MVH3200D products make them an ideal choice for portable and remote applications, or any situation requiring minimal power consumption.

Common Humidity Sensor Applications

MEMS sensors can be used in a huge range of applications, from smartphones to vehicles to residential temperature control systems. Offering highly reliable performance, these tiny sensors help keep countless everyday devices operating accurately and efficiently.

MEMS Vision Sensors From Servoflo

The MEMS Vision line offers built-in, fully calibrated systems, ensuring high-accuracy measurements and linear behavior even in varying sensing environments. These sensors are known for their long-term stability and reliability, with a unique build that includes:

Proprietary sensing structures

Long-term protection technology

Robust biasing circuitry

Self-diagnosis algorithms to ensure accurate, repeatable measurements

Each sensor also features digital output capabilities, allowing for native interfacing with embedded system components, such as FPGAs, microcontrollers, and so on.

To learn more about MEMS Vision products and explore the full catalog of humidity and temperature sensor offerings from Servoflo, download a copy of our new “Humidity and Temperature Model Overview.”

Sensors are rapidly changing with increased functionality in MEMs technologies. The new MS8607-02BA01 exemplifies how new technology is creating amazing new sensors.

The MS8607 measures pressure, humidity and temperature in a small QFN package measuring only 5 mm3 x 3 mm3 x 1 mm3. Inside this small package is a piezoresistive sensor providing pressure and temperature and a capacitive-type sensor for measuring relative humidity. The measured signals are converted into a 24-bit digital value for pressure and temperature and a 12-bit digital value for relative humidity measurement.

The sensor requires 8 pads for operation, uses I2C communication and operates with a very low 1.8 to 3.6 supply voltage. With an operating pressure range of 10 to 2000 mbar, it has a pressure accuracy of ±2.0 mBar and pressure resolution of 0.016 mBar. For humidity, the measuring range is 0-100% rH with an accuracy of ±3% rH and a resolution of 0.04% rH. Finally, for temperature, the operating range is -40°C to +85°C with a temperature accuracy of ±2°C and resolution of 0.01°C.

The MS8607 is ideal for applications in the consumer and industrial marketplace, including smartphones, tablets, printers, HVAC applications, weather stations, home appliances, humidifiers and much more.

The HTU20/21 are tiny humidity and temperature sensors in a 3 mm x 3 mm x 0.9 mm DFN package. This tiny sensor packs a punch due to its:

Humidity operating range of 0-100%

Temperature range of -40°C to +125°C

Supply voltage of 0.3 to 3.6 VDC

Optional PTFE filter (F) option (shown in image on right)

No calibration required in standard conditions, allowing for full interchangeability

Instantaneous desaturation after long periods in saturation phase

Every sensor is individually calibrated and tested. The lot ID is printed on the sensor and an electronic ID is stored in the chip. Resolution can be changed by command from 8/12 bit up to 12/14 bit. A fast response time of 2 ms (using 8 bit resolution) up to 14 ms (using 12 bit resolution) gives designers flexibility in managing their humidity measurement.

For temperature measurement, the accuracy is ±0.3°C at 25°C. The resolution is 0.01°C at 14 bit resolution and 0.04°C at 12 bit resolution.

The HTU20 has an accuracy of ±5% from 20-80% rH while the HTU21 has an accuracy of ±3% in that same humidity range. There are 2 output choices in each family: The P version has a pwm output while the D version has I2C.

Best of all, the price of the HTU humidity sensors is extremely competitive. At 5,000 pieces, the price is between $1.46 to $1.95 each depending on the model.

Applications for the HTU20/21 family include medical, home appliances, printers, humidifiers, smart phones/tablets, indoor air quality and more.

Sensor technology has changed dramatically over the past few years. Previously, customers purchased the sensor elements and electronics and calibrated a sensor themselves. The development of improved electronics, MEMs technology and efficient manufacturing technology has created a new humidity sensor market - fully calibrated humidity sensors with an I2C output. Many companies are offering models with similar features.

Yet, price and performance does vary significantly between models, making it important for users to understand the the performance of a humidity sensor over the entire range. This data is often well hidden in fine print in data sheets.

IST, the supplier of our HYT digital humidity elements and the new sensor elements P14, tested 4 different humidity sensors at 85% rH @ 30C for 65 hours. 2 competitors, the HYT271, and the P14 humidity element were tested. The 2 competitors units showed a 1.25% and 2.5% deviation in readings compared to only 0.17% for the HYT271 and 0.09% for the P14.

What does this mean for designers trying to choose a humidity sensor? The humidity sensor element (such as P14) plays a critical role in the performance of a fully integrated and compensated humidity sensor. Electronics is limited in compensating for poor signals a sensor element sends. The combination of good sensor elements and electronics is the key to sensor design.

The integration of a sensor element and electronics is fairly straightforward. However, if performance over time and at extreme conditions is critical for your application, take the time to research the quality and stability of the sensor element used. Saving a little in upfront component cost could end up costing you much more in the long run!

This article provides technical details about how these problems are solved with new digital humidity sensor technology. In depth details about design features and manufacturing techniques are discussed. Topics include ASIC functionality and design, construction, and what this means for the functionality of the humidity sensor.

Anyone who is considering designing in a digital humidity element should take a quick look at this topic. Please feel free to post a comment or question.